The present invention relates to the art of communicating by radio between plural stations, and more particularly to apparatus for use in communicating data and voice information between a fleet of vehicles and one or more base stations.
Radio communication equipment is often employed to communicate necessary information between the individual vehicles of a fleet of such vehicles (e.g., taxi cabs, delivery trucks, police cruisers, etc.) and their central dispatching station. Although often more than a single radio channel is employed for communicating between these individual vehicles (usually referred to hereinafter as "mobiles") and the central station (usually referred to hereinafter as the "base" station), confusion may nonetheless result when two or more of the mobiles attempt to communicate with the base station at a given time. It would be desirable to keep closer watch on the operation of the mobiles, and to utilize the radio communications network for this purpose. However, this would further complicate the already difficult communications problem associated with maintaining contact with the fleet of mobile stations.
In the past, systems have been devised for automatically communicating desired information from the mobile stations to the base station by means of a time division multiplexing technique. In this technique, each mobile station transmits information to the base station during an associated allotted time slot. Since each of the mobiles transmits only during its own time slot, and since these time slots are not overlapping, it is possible to use a single radio frequency channel for all of the communications.
To successfully accomplish this, of course, it is necessary to synchronize the transmission times for the various mobile stations so that no overlapping of transmissions occurs. In a system disclosed in the patent to Chisholm, U.S. Pat. No. 3,419,865, each of the mobile stations includes a highly accurate, crystal controlled clock which drifts at a very low rate. Due to the low drift rate associated with this clock, adjustments in the clock timing to maintain system synchronization are only infrequently required. When clock timing adjustment is required, however, it is necessary for the operator to manually advance or retard the timing of the clock so as to appropriately adjust the transmission time slot.
In another system currently in operation in Sweden, the base station periodically transmits a synchronizing signal which synchronizes the clocks included in each of the mobile units, thereby automatically readjusting their timing. This approach is preferable, since it does not require intervention by the operator of the mobile unit.
There are occasions in which the desired service area of the communication system is greater than the transmission range of one base station. To properly cover this service area, it is necessary to include two or more base stations. This substantially complicates the problem of communicating with the plural mobile units of the fleet. Thus, if all of the base stations use the same frequency, their transmission will interfere. The Chisholm method of using permanently assigned, non-overlapping time slots may still be used, however this provides inefficient use of the channel, lacks flexibility, and permits only one way communication. If, on the other hand, each base station has a unique associated frequency channel, then the mobile stations (which may move freely from the service area of one base station to another), must include provisions for switching their radio communications between the plural RF channels.
An additional problem relates to the entry of new mobile units into the field. If the Chisholm method of permanently assigning each of the mobile units a specified time slot is used, it will always be necessary to provide as many time slots as the maximum number of vehicles which can ever be fielded. It would be preferable to allot a time slot to a given vehicle only upon its entry into the field. In this case, however, some method must be provided for entering the new vehicle into the communication cycle without interfereing with the communications taking place between the other vehicles already participating in the net.